The present invention relates to force-balanced capacitive transducers including sensors and actuators. More specifically, it relates to a method to electrostatically force-balanced transducers that control the position of a surface, stylus, inertial mass, valve, electrical contact, electrical component; or an optical component such as a mirror, lens, grating, filter, or holographic element.
Electrostatically, force-balanced, capacitive transducers maintain a compliant member, such as a beam, diaphragm, or bridge at a fixed position or predetermined gereratrix. One example is an accelerometer with capacitor electrodes that sense the displacement of an inertial mass suspended on a compliant member in relation to stationary support structure. A simple capacitive accelerometer measures a change in spacing between a capacitor electrode affixed to a surface of an inertial mass in close proximity to a cooperating capacitor electrode affixed to adjacent stationary structure. The two electrodes form a displacement sensing, parallel-plate capacitor with a capacitance value inversely proportional to the plate spacing. A force acting on the mass causes it to be displaced in proportion to the stiffness of the compliant member. When an unconstrained axis of the mass is orientated with the gravity gradient, it will deflect to an equilibrium or quiescent position where restoring forces due to the bending moment of the compliant member balance the force of acceleration acting on the mass. Motion of the support structure modulates the capacitor plate spacing due to the inertia of the mass. The corresponding change in capacitance is detected and transduced to provide a voltage proportional to the square root of a change in force acting on the mass.
The disadvantages of a simple capacitive accelerometer are: 1) a severely restricted dynamic measurement range due to a small capacitor gap, 2) a highly non-linear capacitance response with mass displacement, and 3) the requirement to dampen the response of the accelerometer at frequencies near the resonance of its spring-mass system. These disadvantages are avoided by operating a capacitive accelerometer in an electrostatic force-balanced feedback loop that maintains the inertial mass in a substantially fixed position under loading. An incremental displacement of the mass is capacitively detected, transduced, and amplified to provide a control voltage V across the capacitor plates to create an electrostatic force that restores the mass to its initial position. The electrostatic force FE applied to a electrode of a parallel-plate capacitor varies as V2/d, where d is the effective plate spacing. Because the force FE is independent of the polarity of the control voltage V, the compliant member of a simple capacitive transducer is required to be electrostatically biased or mechanically preloaded to allow the accelerometer to sense a bi-directional force. Accelerometers, actuators, and other capacitive transducers with one, force balancing capacitor are defined here as single-side, force-balanced transducers.
In a similar manner, a simple electrostatic actuator applies an electrostatic force to an electrode affixed to a compliant member and/or attached payload to control the position of the member or payload. The payload can include a surface, stylus, inertial mass, valve, electrical contact, electrical component; or an optical component such as a mirror, lens, grating, filter, or holographic element. This method is of controlling the position of a compliant actuator member is equivalent to applying a voltage bias to the electrodes of a capacitive accelerometer to move its inertial mass to a new equilibrium position. For most actuator applications, a static force such as gravity is generally small compared to the applied electrostatic force and the reaction force of the compliant member and payload. A general disadvantage of a single-side, capacitive transducers (e.g., sensors and actuators) is that the effective spring constant and elastic restoring force of the preloaded compliant member is influenced by ageing and physical effects such as temperature.
Precision capacitive transducers use at least a pair of differential parallel-plate capacitors to sense and force-balance an inertial mass or to sense and position of a compliant member of an actuator. Planar electrodes are affixed to opposing surfaces of the mass and/or a compliant member which are located in close proximity to cooperating planar electrodes affixed to adjacent stationary structure. The electrodes form one or more pairs of differential displacement sensing and force generating capacitors. When the mass of an accelerometer or payload of an actuator moves, the capacitance of one displacement sensing capacitor increases while that of a second capacitor decreases by substantially an equal amount. One advantage of a differential capacitive transducer is that the compliant member is not required to be preloaded for bi-directional force or position control. Another advantage is that an incremental capacitance change can be detected with a bridge circuit to minimize errors associated with unmatched electronic components and variations of supply and reference voltages.
Capacitance transducers with variable-gap, parallel-plate electrodes comprise a well-know and crowded art. The general disadvantage of prior-art capacitive transducers arise from the limitations imposed by parallel-plate capacitors: low-quiescent capacitance, low capacitance-load sensitivity, non-linear response, and the requirement to form and accurately maintain a precision structure with narrow electrode spacing. Variable-area capacitance transducers with a flexible electrode responsive to a physical effect are less known and less appreciated for their ability to provide an order of magnitude and greater increase in quiescence capacitance, capacitive-load sensitivity and linear dynamic range. Variable-area capacitors of U.S. Pat. No. 6,151,967 and those simply illustrated in FIGS. 1 and 2 are constructed by sandwiching a thin dielectric layer between a flexible electrode and a cooperating rigid electrode with a curved surface. The dielectric layer maintains a region of fixed capacitance spacing between mutually opposed areas of the rigid electrode and flexible electrode. The region of fixed capacitive spacing increases in extent as the flexible electrode deflects in response to a physical effect. The flexible electrode can be of electrically conducting material, or it can be comprise an electrically conducting layer affixed to at least one surface of a compliant member of dielectric material.
A variable-area capacitor with a flexible electrode comprising a compliant cantilever beam was described by Carter et al., xe2x80x9cMeasurement of Displacement and Strain by Capacity Methodsxe2x80x9d, Proc. J. Mech. Engr. (152) 1945, pp. 215-221. The Carter transducer is generally of the design shown in FIG. 1. The use of this transducer as a displacement sensor was described by Frank, Electrical Measurement Analysis, McGraw-Hill, NY, 1959. Variable-area capacitors of U.S. Pat. No. 6,151,967, can be fabricated with flexible electrodes that include circular and rectilinear diaphragms. Transducers of this general design are in FIGS. 2 and 3.
The electrostatic deflection of a cantilever beam is a well known art that was recently reviewed by Legtenberg, et al., xe2x80x9cElectrostatic Curved Electrode Actuators,xe2x80x9d J. Micro Electro Mech Syst. vol. 6, no. 3, 1997. The electrostatic deflection of circular and rectilinear diaphragms for a loudspeaker is an invention of Kyle, U.S. Pat. No. 1,644,387, Oct. 4, 1927. The Kyle invention is further discussed by Ford, et al., xe2x80x9cThe Kyle Condenser Loud Speaker,xe2x80x9d P. Inst. Radio Engr., vol. 17, no. 7, 1929. Since this early work, a rich art has developed for electrostatically controlled actuators. None of the above cited prior-art references teach or suggest using a variable-area capacitor to develop an electrostatic force to position an actuator and the same capacitor to sense the position of the actuator for closed-loop electrostatic position control.
Accordingly, an object of the present invention is to provide a method to control the displacement of a flexible electrode of variable-area capacitive sensors and actuators by electrostatic force feedback.
The transducer of Cadwell, U.S. Pat. 4,584,885, and other early capacitive transducers used a first pair of capacitors to sense the displacement of a mass and a second pair of capacitors to apply an electrostatic force to the required side of the inertial mass to maintain it in a substantially fixed position. The disadvantage of this arrangement is that the surface area of the inertial mass is divided to accommodate two, smaller area capacitor electrodes. The accelerometer of Sherman et al., U.S. Pat. No. 5,540,095, and other force-balance capacitive transducers, are constructed with electronics that allow a single pair of capacitors to be used for both displacement sensing and force-balancing. The accelerometer of Suzuki et al., U.S. Pat. No. 5,095,752, and Yazdi, et al., U.S. Pat. No. 6,035,714, utilize only three electrodes to perform the displacement sensing and force balancing function. For these accelerometers, two differential capacitors are formed by affixing two planar electrodes on rigid support structure adjacent to opposite sides of an inertial mass. If inertial mass is of electrically conducting material, opposing surfaces of the mass serve as a common movable electrode for both capacitors. Alternately, electrodes affixed to opposing surfaces of the mass can be connected electrically in parallel to provide a common electrode. A disadvantage of prior art, differential capacitive transducers and associated capacitance detection electronics is that a common electrode cannot be electrically grounded. Grounding the common electrode minimizes the susceptibility of a transducer to electromagnetic interference and signal loss due to the attenuation (e.g., signal division) of stray capacitance. The total capacitance of 0.1 pF disclosed for the accelerometer of Sherman is representative of the values of other micromachined capacitive transducers. This value is small compared to the input capacitance of active electronics devices used for signal amplification.
Accordingly, still another object of the present invention is to provide a method that allows an electrode of a capacitive transducer controlled by electrostatic feedback to be grounded.
Variable-area capacitance sensors and actuators can be fabricated by many of the methods used to construct parallel-plate capacitive transducers with planar capacitor electrodes. U.S. Pat. No. 5,095,752, Suzaki et al., is one example of an accelerometer with two stationary planar electrodes and a common movable electrode. The movable electrode includes a thin section connected to an inertial mass that is bulk micromachined from single-crystal silicon. The invention of Sherman is a second example of a capacitive accelerometer with stationary planar electrodes and a common movable electrode. This accelerometer is surface micromachined by the well known steps of sacrificial layer etching. The profile and sidewalls of the movable electrode of polycrystalline silicon is formed over a sacrificial layer, typically of silicon dioxide (SiO2) or oxynitride (SiOXNX) formed on a silicon substrate. The movable electrode is then released by etching the underlying silicon dioxide with hydrofluoric acid. U.S. Pat. No. 6,199,871 B1, Galvin et al., is a third example of a capacitive accelerometer with planar electrodes formed by surface micromachining of silicon using the method of multi-step, deep reactive ion etching (DRIE). The details of these and other micromachining methods are described Elwenspoek, et al., Silicon Micromachining, Cambridge University Press, Cambridge, UK, 1998.
Accordingly, another object of the present invention is to provide a method to control by electrostatic force feedback the displacement of a flexible electrode of a variable-area capacitor fabricated at least in part by the processing steps of bulk and surface micromachining.
The above cited capacitance sensors and actuators require a means to detect an incremental change of capacitance between at least two electrodes or a pair of differential capacitor electrodes. These inventions are representative of the many different capacitive transducers that can exploit the advantages of the capacitive detection circuits of U.S. pat. application Ser. Nos. 09/834,691 and 09/794,198. Specific advantages include: high DC stability, low impedance inputs, wide dynamic range, a linear output, the ability to ground a common capacitor electrode, and an option to provide a high-resolution digital signal output. The advantages of both circuit inventions are realized by variable capacitors with variable-area electrodes, variable-gap electrodes, and a combination of variable-area and variable gap electrodes. A further advantage of these preferred circuit inventions is that a bridge excitation signal applies a small periodic vibratory force to a flexible electrode to overcome and average any micro-frictional and stiction forces. The frequency of the vibratory force is higher than the frequencies of forces being detected or motion being controlled.
Accordingly, a further object of the present invention is to provide a method to control the displacement of a moveable electrode of a capacitive sensor or actuator using a capacitance detection circuit that provides current feedback to actively null a bridge network used to provide a voltage to electrostatic force balance the movable electrode.
The present invention is a method of providing electrostatic force feedback to maintain the displacement of a movable electrode of a variable capacitor in fixed relationship with a cooperating stationary electrode.
A preferred embodiment is a method to control a flexible electrode of a capacitive transducer by electrostatic force feedback comprising the steps of providing a capacitor with a flexible electrode and at least one cooperating stationary electrode; providing a capacitance detection circuit to detect a value of capacitance between the flexible and at least one stationary electrode; measuring and amplifying a change in capacitance of the capacitor to generate a feedback control voltage; and applying the feedback control voltage across the flexible and at least one stationary electrode to maintain the flexible electrode at a fixed position or predetermined generatrix. The compliant electrode can be a beam, diaphragm, or flexible bridge that supports a payload.
This invention exploits the advantages of variable-area capacitive sensors and actuators: high quiescent capacitance and capacitance-load sensitivity and the ability to displace a flexible electrode at low voltages compared to the voltage required to move a planar electrode of a parallel-plate capacitor. One aspect of the method of the present invention includes providing a capacitive transducer and a capacitance detection circuit that allows one of the two electrodes of a single-side variable capacitor to be grounded. For the preferred capacitance detection circuit of the present invention, one electrode of a variable capacitor is connected to a ground potential and a second terminal is connected to a first-side node of an actively nulled capacitive bridge. A reference capacitor is connected between a second-side node of the bridge and said ground potential. Alternately, capacitive changes can be detected with less accuracy using an actively nulled, half-bridge network connected to a reference voltage. For transducers with a pair of differential variable capacitors, one electrode of each capacitor is connected to opposing sides of an actively nulled bridge. Operational descriptions and optional designs are disclosed for full bridge and half bridge networks in U.S. patent application Ser. Nos. 09/482,119, 09/794,198, and 09/816,551.